This invention relates to a composit superior in corrosion resistance and having a structural member provided on its surface with a film layer of an amorphous material and a method for producing the composite member.
Various methods have been known to improve the corrosion resistance and gas emission property of metallic structural members. For example, surfaces of a structural member are treated by electropolishing or formed with a passive film, or formed with a film such as amorphous SiC, amorphous SiN or the like.
Among the above known methods, in order to form passive film, a structural member of a stainless steel is heated in an oxidizing atmosphere. In case of a structural member of aluminum, it is treated by fluorine to form a passive film.
However, the passive film formed by oxidizing the surface of the stainless steel is very thin on the order of a few hundred .ANG., so that a corrosive material penetrates into the film in a short period of time to shorten the service life of the composite member. Moreover, in order to improve the corrosion resistance, even if a surface treatment is performed for increasing the ratio of the number of chromium atoms, such a treatment has a limitation in effect because the oxidized surface film includes iron oxide and chromium oxide without exception.
On the other hand, the film layers of amorphous materials do not encounter the above problems. However, in the event that the film layer of an amorphous SiC, SiN or the like is formed on the surface of a metallic structural member which has been worked in a usual manner by cutting with a lathe, milling machine or the like, some crystal grains of the metallic structural member are likely to fall off in grinding after the cutting. Therefore, pin holes are formed in the film layer at the locations from which the crystal grains have fallen off. Under the existence of the pin holes, there is a tendency for the corrosive-resistant film layer to be peeled away from the structural member in conjunction with the action of corrosives.